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The future international facility FAIR (Facility for Antiprotons and Ion Research) will deliver beams of all ions up to uranium with intensities of up to 1012 ions/s. At FAIR it will be possible to perform experiments in different fields of physics. A wide range of ions with energies up to 1.5 A GeV will be used for the production of fragments by projectile fragmentation/fission at the in-flight fragment separator, Super-FRS. Rare isotopes of all elements up to uranium will be produced and spatially separated within a few hundred nanoseconds, enabling the study of very short-lived nuclei. The Super-FRS is a large-acceptance device utilizing large-aperture superconducting magnets and employing multiple degrader stages to provide monoisotopic nuclear beams. It serves different experimental branches including a new storage-cooler ring system. Because of high primary beam intensity a challenging design of the target area and the components used in the first dipole stage of the Super-FRS is necessary. These efforts include high-power production targets, beam dumps to catch the remaining primary beam and radiation resistant magnets. In this contribution we present the status of the project.

Investigations of specific heat and T c under pressure are presented for YBa 2 Cu 3 O x (Y123), NdBa 2 Cu 3 O x (Nd123) and Y 1−y Ca y Ba 2 Cu 3 O x (YCa123) for various oxygen contents. In Y123, at the maximum of the specific-heat jump ΔC/T c the pressure dependence of the transition temperature dT c /dp shows a zero crossing. Inversely, at the maximum of dT c /dp the jump in the specific heat becomes minimal. With Ca doping these joint features shift to lower oxygen contents x, whereas Nd123 causes a shift into the opposite direction. Considering the hole doping by Ca and the larger chain fragmentation in Nd123, we rescaled our results with respect to the hole content n h in the CuO 2 planes. In this plot features coalesce in two fixed hole concentrations, n h ≈0.11 and n h 0.17, irrespective of the substitution and doping. The maximum in ΔC/T c may be related to the appearance of a quantum critical point due to the disappearance of a pseudogap for hole contents above n h ≈0.17. The features at n h ≈0.11 point to a drastic change in system properties at this hole content that may be connected to various anomalies found in other properties reported in the literature.

Westudied T c of Y Ba 2 Cu 3 O x (Y123), Y 0.89 Ca 0.11 Ba 2 Cu 3 O x (YCa123) and NdBa 2 Cu 3 O x (Nd123) single crystals with various oxygen contents x. Compared to T c (x) of Y123 the T c (x) curves of YCa123 are shifted to lower oxygen contents and the maximum transition temperature T c,max decreases with increasing Ca content whereas in Nd123 T c (x) is shifted to higher oxygen contents and T c,max is increased. According to the universal parabolic T c (n h ) behavior the differences in T c (x) of Y123, YCa123 and Nd123 can be ascribed to different hole concentrations n h in the CuO 2 planes caused by doping via changes in chemistry or structure. In order to study the influence of structural changes on T c we examined the hydrostatic pressure effect dT c /dp (p ≤0.6GPa). In the underdoped region, at n h ≈0.11, the examined compounds show a peak in dT c /dp which is very pronounced for systems with well ordered CuO chains. As this peak occurs at the same n h in all investigated systems it is not caused by oxygen ordering, but its origins might be found in a strong influence of lattice deformations on the electronic structure.

In YBa 2 Cu 3 O x (x < 7.0), above 225 K a oxygen ordering in the CuO chains is visible as a glass-like transition in thermal-expansion measurements. The large contribution of this process to the thermal expansion and the T c changes observed under application of external hydrostatic pressure point to a strong dependence of the oxygen ordering and its activation energy E a on the unit-cell volume V. If, however, V is changed chemically by replacing Y with other rare earth ions, R, the values of E a reported in the literature show only a slight dependence on V. We have measured the thermal expansion of a YBa 2 Cu 3 O 6.94 single crystal under hydrostatic pressure up to 0.5 GPa. The activation energy at ambient pressure was determined to be 1.0 ± 0.1 eV with a pressure dependence dE a /dp = 78 meV/GPa. The volume dependence dE a /dV = -48 meV/A 3 obtained from these pressure experiments is clearly higher than dE a /dV = -5.4 meV/A 3 from R-ion substitution. Thus we conclude that a non-uniform compression of interatomic distances within the unit cell has to be considered to explain the difference between dE a /dV(p) and dE a /dV(R). The contribution, Δα V , of the ordering process to the thermal-expansion coefficient, α V , increases with pressure, indicating that more and more oxygen atoms take part in the process. From an extrapolation to negative pressure via R-ion substitution we expect a vanishing (R=Nd) or inverted (R=La) pressure effect on oxygen ordering.

In YBa2Cu3Ox, distinct features are found in the pressure dependence of the transition temperature, dTc/dp, and in DeltaCp*Tc, where DeltaCp is the jump in the specific heat at Tc: dTc/dp becomes zero when DeltaCp*Tc is maximal, whereas dTc/dp has a peak at lower oxygen contents where DeltaCp*Tc vanishes. Substituting Nd for Y and doping with Ca leads to a shift of these specific oxygen contents, since oxygen order and hole doping by Ca influences the hole content nh in the CuO2 planes. Calculating nh from the parabolic Tc(nh) behavior, the features coalesce for all samples at nh=0.11 and nh=0.175, irrespective of substitution and doping. Hence, this behavior seems to reflect an intrinsic property of the CuO2 planes. Analyzing our results we obtain different mechanisms in three doping regions: Tc changes in the optimally doped and overdoped region are mainly caused by charge transfer. In the slightly underdoped region an increasing contribution to dTc/dp is obtained when well ordered CuO chain fragments serve as pinning centers for stripes. This behavior is supported by our results on Zn doped NdBa2Cu3Ox and is responsible for the well known dTc/dp peak observed in YBa2Cu3Ox at x=6.7. Going to a hole content below nh=0.11 our results point to a crossover from an underdoped superconductor to a doped antiferromagnet, changing completely the physics of these materials., Comment: 6 pages, 5 figures Proccedings of the 'Stripes 2000' Conference, Rome (2000)